GB1287598A - Communication system - Google Patents

Abstract

1287598 Electric signalling systems INTERNATIONAL BUSINESS MACHINES CORP 11 Dec 1969 [14 Jan 1969] 60457/69 Heading G4H In a communication system a transmitter transmits a frequency-modulated signal, and each of a plurality of receiving stations incorporates a frequency demodulator 30 for demodulating the transmitted signal, the demodulator at each station having means to clamp the output within a preselected range so that the demodulator output varies only for frequency variation of the transmitted signal within a corresponding preselected range, the ranges being different for each of the receivers. The demodulator converts the incoming frequency signal into an amplitude signal linearly related to the frequency. This is applied to a pair of clamps 40, 50, the clamping level of which is set by potentiometers 42, 52. Clamp 40 sets an upper level so that if the signal is above this level a constant maximum voltage is produced at point 33. If it is below a certain level clamp 50 sets a constant minimum voltage at 33. If the level changes within this range a corresponding changing voltage is produced at 33. The signal at point 33 for three receivers is shown in Fig. 4. The signal at 33 is differentiated at 60 and the output is applied to single shots 61, 62 responding to signals of different signs. A frequency transition from f1 to f2 causes the first station S1 to indicate a binary "0" and the opposite transition, a binary "1". The other stations do not respond.